Well, the mystery continues...

My mechanic tested the coolant (while the engine was both hot and
cold) for traces of combustion leakage (hydrocarbons in the cooling
system) and found nothing.

He also found no excessive pressure in the oooling system (I replaced
the expansion tank pressure cap 2 weeks ago, just to be safe).

No explanation for 4 rear heater cores and 2 differential coolers
bursting in one year!

He is suggesting it could this be an intermittent problem??? I don't
want to lose another transmission!

Any ideas from my Vanagon.com friends would be welcome.

Thanks,
Paul